The present invention relates to identification and quantification of radionuclides using a radiation detector. In particular, the present invention relates to an apparatus and method for sampling the output signals of a radiation detector and distinguishing signals representing full-capture events for those representing Compton scattering events.
2. Discussion of Background
Conventional radiation spectroscopy techniques are based on the interaction of ionizing radiation with the atoms of a suitable detector. As the radiated particle loses energy in each interaction, it deposits an excess electrical charge in the detector. If all of the charge so deposited were collected (as by integration by a capacitor), then the energy of the incoming particle can be determined. Therefore, present-day instruments attempt to optimize charge collection, i.e. to collect all the excess electrical charge.
The energy of the ionizing radiation particle is a clue to the identity of the source of the particle. A given source may produce, through radioactive decay, particles having one or more energy levels characteristic of that source. To determine the energy spectrum of the incoming particles, the energy produced by each interaction--or event--in the detector is measured, and the number of events that occur having an energy in each energy range are counted and plotted versus energy. FIG. 1 shows a sample spectrum 10. This two-dimensional method of viewing the data is the method used with current instrumentation.
Because events occur to some extent randomly and several events may occur almost simultaneously, collecting, sorting and analyzing the event data can pose an electronic obstacle. Many devices and techniques are available for obtaining event data from detectors. Data-collection systems are typically "event-triggered," that is, the system is enabled when the detector output exceeds a predetermined threshold value. Data collection continues until the detector output falls below the threshold value, whereupon the system is returned to its normal or "non-event" state.
"Dead-time" limitations are inherent in event-triggered systems: once the system is triggered by the start of an event, it cannot collect or process subsequent events until the system has returned to its normal or "non-event" state. Therefore, event-triggered systems have limited ability to distinguish between separate, nearly simultaneous events. When an event enables the system, a later event may begin before the first event is completed and before the system can return to its normal state. Thus, the system will not resolve the signals of the two separate events; the two events will be treated as a single, continuous event of extended duration.
A method and apparatus for reducing dead-time are described in commonly-assigned U.S. patent application Ser. No. 08/014,916, filed Feb. 8, 1993; ("Method and Apparatus For Data Sampling"). In this application, the detector output is continuously sampled at a high rate. When no event is occurring, the sampled data represent only "non-events" including noise. When an event is occurring, the data represent "events," that is, the interaction of particles with the detector. The sampled data are encoded as binary numbers, or "digitized," and processed to identify those samples that are representative of detected events. The use of continual, high-speed digital sampling reduces both deadtime and also other problems associated with analog data-collection techniques, such as drift.
But nether the traditional, event-triggered method nor that disclosed in application Ser. No. 08/014,916, ("Method and Apparatus For Data Sampling"), can distinguish "full-capture" events, where all the energy of an incoming particle is deposited in the detector, from "partial-capture" or Compton scattering events of the same energy, where only some of the particle's energy is deposited in the detector. When integrated in the traditional manner or according to the technique described above, a Compton event is indistinguishable from a full-capture event of the same energy. Thus, Compton scattering events often obscure full-capture events of comparable energy, leading to inaccuracies in data analysis and evaluation.
Several techniques are available for correcting spatial information in radiation imaging systems such as gamma cameras and scintillation cameras. Barfod (U.S. Pat. No. 4,899,054) reduces the sensitivity for some locations on the detector of a gamma camera system to compensate for inherent non-uniformities in the system. Del Medico, et al. (U.S. Pat. No. 4,316,257) and Knoll, et al. (U.S. Pat. No. 4,212,061) use stored spatial distortion correction factors to correct for distortion effects in scintillation cameras. These spatial-discrimination techniques are not capable of correcting time-based data, including distinguishing Compton scattering from other detector events.
In U.S. Pat. No. 4,258,428, Woronowicz discloses a gamma camera having a Compton scattered radiation de-emphasizer. Scintillation events are displayed at the (x,y) coordinates of the event on a cathode ray tube screen by unblanking the tube with z pulses applied to its control electrode. The de-emphasizer sends small z pulses into the part of the spectrum where Compton scattering is most prevalent and increasingly larger z pulses where Compton scattering is insignificant. The system produces only a qualitative display, using predetermined, empirical correction factors. But individual Compton events as such are not distinguished from full-capture events by his system. Some multiple-detector systems use time coincidence to distinguish Compton events from full-capture events. However, no presently-available single-detector system can distinguish full-capture events from Compton scattering events.